Pearlitic gray cast iron (120-250 BHN) is a widely used material in many automotive components. In spite of the relative ease of machining of this material, variations in tool-life can lead to expensive downtimes and tool-costs. Polycrystalline cubic boron nitride (PCBN) cutting tool materials successfully machine gray cast iron at very high speeds of ˜1,000 m/min. The actual magnitude of tool-life measured in cumulative minutes of cutting is rather high (˜100 min). End of tool life is usually determined when either the finished part does not possess the desired dimensions or surface finish for set process parameters. Both occur when tool wear on the insert is higher than normal.
However, rising productivity requirements and lower capital expenditure are driving the need for greater tool-life in established machining operations. Moreover, variability in PCBN tool-life due to the age of the casting between shakeout from the mold and machining is also a consideration. An increasing number of cases are being reported in the industry where tool-life has dropped below the usual limit (100%) on the number of parts set for tool-life when the age of the casting is less than 100 hrs. Furthermore, in cases where a negative cutting geometry is employed, solid PCBN inserts for machining are more economical than carbide-supported geometries. This is usually due to lower insert cost per part accounting for down-time for tool changes and also due to greater number of cutting corners per insert in a solid PCBN insert. Efforts to tailor the physical and thermochemical properties of the PCBN insert material and coatings on the insert have been underway for many decades in the cutting tool industry, buts success however depends on sound understanding of the fundamentals of machining and of the application.
During machining, in addition to cutting geometry and process parameters, all thermophysical and physiochemical properties of the cutting tool material are brought to bear in determining tool life and performance. The cutting zone is one of high local temperatures and moderately high pressures and stresses. Generally, lower cutting zone temperatures and chip-tool interface temperatures promote greater tool-life in gray cast iron machining, but only a small proportion (˜5-10%) of the heat generated in the chip is transmitted into the cutting tool. This small proportion also decreases as cutting speed increases. From this standpoint, given that gray cast iron is machined at high cutting speeds, it is not expected that even drastic changes in thermal properties of the cutting tool will produce a commensurately lower chip-tool interface temperature. In a few other machining applications also, it can be seen that tool life of ceramic cutting tool materials like PCBN is less sensitive to cutting tool temperature. Therefore, an undertaking to improve thermal transport properties such as diffusivity (hereinafter referred to as κ) and conductivity of the PCBN, among its various physical properties, for machining of gray cast iron does not seem like an obvious choice. Two mechanisms of thermal transport are electron scattering and photon scattering.
Sonic velocity (hereinafter v) is another physical property of ceramics. As measured using the scanning acoustograph instrument it records the speed of longitudinal compression waves in the sintered solid. PCBN, owing to its high Elastic modulus (˜720 GPa) and low density (˜3.5 g/cm3), possesses a very high v. PCBN compacts are normally isotropic in nature and devoid of residual porosity. It may generally be said that v is a measure of the contiguity of the microstructure to transmit normal and shear tractions. Consequently, binder content, binder chemistry and multi-modality, i.e., the presence of a multiplicity of peaks in the grain-size distribution are all encapsulated in v measurements. In the current invention, the relation of v to κ of solid PCBN compacts is both revealed and utilized. Sintering conditions and sintering constituents are tailored specifically to increase room temperature v of the sintered compact. This has the desirable result of increasing κ over a prescribed range of temperature.
In the current invention, the scope of application of inserts made from the PCBN compact is continuous, dry gray iron machining operations. Thermal cracks and chipping of the cutting edge do not frequently occur in dry continuous gray cast iron machining because the workmaterial advancing to the cutting edge is considerably softened and the cutting forces are relatively stable. The coefficient of thermal expansion (CTE) is critical to machining performance only when the cutting edge undergoes rapid cyclical expansions and contractions, not applicable to the scope covered in this invention. FIG. 1 shows an example of a solid PCBN insert worn from continuous machining of gray cast iron. The thermal cracking mode of edge damage, as described earlier, is completely absent.